1. Field of the Invention
This invention relates generally to a method for estimating the maximum tire/road surface coefficient of friction for a vehicle and, more particularly, to a method for estimating the maximum tire/road surface coefficient of friction for a vehicle that includes inducing an acceleration in the front wheels or the real wheels of the vehicle and inducing a deceleration in the other of the front wheels or the rear wheels of the vehicle so that the acceleration and deceleration cancel and the perceived speed of the vehicle does not change.
2. Discussion of the Related Art
The performance of various vehicle control systems, such as collision avoidance systems, adaptive cruise control systems, automated lane-keeping systems, automated braking systems (ABS), traction control systems (TCS), stability control systems, etc., can be greatly improved by providing a reliable estimate of the maximum tire/road surface coefficient of friction. For example, the gains and the commanded values of the vehicle control system can be made adaptive to the maximum tire/road surface coefficient of friction to increase the performance of the system.
The wheel-slip of a vehicle wheel during acceleration is defined as the difference in wheel speed between a driven wheel and a non-driven wheel. The wheel-slip ratio is the ratio of the difference between the speeds of the driven wheel and the non-driven wheel to the speed of the driven wheel. It can be shown that the slope of the wheel-slip ratio road surface coefficient of friction curves for small slip ratios (in the linear region) is almost the same for all road surfaces.
It is typically difficult to classify the road surface condition when the wheel-slip ratio is in the linear operating region. As the wheel-slip ratio increases, the slope for the different road surface conditions starts to decrease and become different. However, once the vehicle is in the non-linear operating region, it may be too late for the control system to utilize the wheel slip information and adapt to the current road surface conditions. Therefore, it is desirable to estimate the maximum tire/road surface coefficient of friction before the wheel-slip ratio reaches the value that corresponds to the maximum coefficient of friction. By estimating the slope of the curve at higher wheel-slip ratios, it is possible to classify the road surfaces as, for example, icy, snowy, wet or dry.
United States Patent Application Publication No. 2007/0061061, assigned to the assignee of this application and herein incorporated by reference, discloses a method for classifying a road surface condition by estimating the maximum tire/road surface coefficient of friction for a vehicle that includes actively inducing an acceleration or deceleration of the vehicle. The induced acceleration/deceleration allows the speed of the wheels to be measured so that the tire/road surface coefficient of friction and the driven wheel-slip ratio can be calculated. The tire/road surface coefficient of friction and the wheel-slip ratio are then used to determine the slope of the wheel slip/coefficient of friction curve to classify the road surface condition.
Although the method of the '061 application is effective for classifying the road surface condition by determining the maximum tire/road surface coefficient of friction and the wheel-slip ratio, the acceleration or deceleration that is induced on the vehicle can be felt by the vehicle operator.